This invention relates to composite semiconductor devices, particularly to one integrally comprising a semiconductor rectifier and unipolar field effect transistor, altogether functioning as diode. More particularly, the invention is directed to such a diode-like semiconductor device which has an improved recovery time, antivoltage strength, and forward voltage requirement.
Inverters (DC-to-AC converters) and switching-mode power supplies are alike in incorporating a switching device that rapidly turns on and off. The voltage applied to the switching devices for their on-off control is so high that the diodes put to combined use therewith must necessarily have a correspondingly high antivoltage strength and short reverse recovery time. These requirements are met to some extent by the known silicon pin (p-type/intrinsic/n-type) diode, or fast recovery diode (FRD) in more common parlance. But even the FRD is not exactly zero in recovery time, and silicon FRDs capable of withstanding voltages up to 600 volts, for instance, necessitate a relatively high forward voltage (curve C in the graph of FIG. 4A).
The Schottky barrier diode (SBD) is also known which has no reverse recovery time due to minority carrier storage. A low-antivoltage-strength silicon SBD requires a relatively low forward voltage (curve B in FIG. 4A) but is incapable of withstanding as high a voltage as required in the applications now under consideration.
Japanese Unexamined Patent Publication No. 2004-22639 suggests gallium nitride (GaN) and silicon carbide (SiC) SBDs which are both capable of withstanding voltages up to 600 volts or so. This advantage is offset, however, by their high forward voltage requirements (curve D in FIG. 4A). It might be contemplated to design these SBDs for a low forward voltage drop, but then they would also in inconveniently low in withstanding capability due to a rise in current leakage in response to a reverse voltage.